This paper deals with boiler control systems which are important for the functional and secure operation of boilers. In this paper, the boiler control systems are shown and described through functional diagrams using only basic principles and terms which are necessary to be known in order to get basic insight into functions of the boiler control system. The main feature of the boiler control system is a control loop or closed circle which main element is regulator or regulation device. In this paper, the control system of four basic boiler control systems is presented: furnace pressure control, feedwater control, fuel and air control and steam temperature control.